Many oil wells are subject to greatly reduced natural production as a result of heavy, low gravity crude oil deposits being combined with low porosity sands in the producing formation. In such shallow formations, crude oil is in equilibrium, and no natural differential pressure exists to drive the crude oil up to the surface. As a result, when crude oil is removed from a recovery well, high viscosity of the oil and low porosity of the formation prevent migration of the crude from the surrounding formation into the recovery well. The high pressure that exists in many deep wells usually does not exist in the shallower wells, such wells being within about one thousand feet of the surface. There are, however, many such stabilized, heavy crude oil deposits to be found in shallow formations. An example of such a heavy crude oil deposit can be found in the shallow San Miguel formation in the central part of Maverick County, northeast of the town of Eagle Pass, Texas.
Structurally, the shallow San Miguel formation is located on the northern, or tip portion of the Chittim anticline. The Olmos formation and underlying San Miguel formation outcrop over the northern portion of the Chittim anticline. This circumstance results in the oil-bearing reservoir sands within the shallow San Miguel formation lying as close as forty feet below the surface of the earth in many areas.
The San Miguel sands are estimated to contain in excess of one hundred one million barrels of heavy crude oil low in sulfur and ranging in gravity from about 10.degree. A.P.I. to 20.degree. A.P.I. depending upon depth. About half the heavy crude lies within one hundred feet of the surface of the earth.
Two methods have been used in an attempt to remove crude oil from the San Miguel sands. An in situ combustion project was attempted. Combustion was supported by injecting air through input wells. The project was abandoned when no significant quantity of oil was produced. In addition, steam injection was attempted with similar dissappointing results.
The prior art includes a number of patents related to the recovery of oil. For example, U.S. Pat. No. 2,286,724 (Garrison) discloses the heat treatment of an oil well by first heating wet sand above the producing formation to the boiling point of water to dehydrate the sand around the well. Garrison proposes heating to be done by an electric heater lowered into the well. Following deydration, the sand grains are coated with an oil wettable material. This converts an oil well that has contained water wet producing sand to one that is wettable by oil, and thereby increases the ratio of oil to water in the producing sand. By dehydrating the wet sand, oil can flow through the capillaries and crevices of the producing sand.
U.S. Pat. No. 3,385,359 (Offeringa, 1968) discloses a method for producing oil from a sub-surface producing formation that consists of very high viscosity tar-bearing sands. The method consists of injecting hot water into the formation, waiting for the formation to heat, then injecting steam and other condensable liquids into the formation. The viscosity of the tar is lowered, and it can be removed. Offeringa also discloses fracturing to reduce the pressure required to inject the water into the formation.
U.S. Pat. No. 3,372,750 (Satter et al., 1968) discloses a method directed to an environment in which oil sands overlay water-bearing sands. In Satter, heat is applied to a large volume of oil while steam is simultaneously used to a reduce viscosity and force migration to a recovery well.
Yet another method of enhancing crude oil recovery using heat is disclosed in U.S. Pat. No. 3,441,083 (Fitzgerald 1969). This method uses both steam drive and in situ combustion techniques in combination with a specific spacing between air and steam injection wells. In Fitzgerald, production wells surround a plurality of steam and air injection wells. Heat and pressure are created by in situ combustion at the top of the formation. Heat and fluid flow is initiated from the injection well to the production well by steam injection in the lower portion of the producing formation. This, in conjunction with gravity and thermal expansion, causes the crude oil to flow downward to the production wells. This method also anticipates fracturing of the producing formation near the bottom before steam injection.
Prior art also discloses processes for recovery of liquids other than liquid hydrocarbons from underground. For example, U.S. Pat. No. 3,759,328 (Ueber 1973) features a method for removing hydrocarbons from oil shale. This method discloses an underground cavern which is continually evacuated and in which shattered and crumbled oil shale is pyrolyzed, releasing hydrocarbons thereby. Pumps are used to inject steam and hot liquids, as well as to remove gas and fluid from the pyrolyzed shale in the underground chamber.
U.S. Pat. No. 461,445 (Monjeau 1891) discloses a process for forming subterranean filtering chambers for the recovery of clean water. The invention relates to enlarging subterranean areas to increase the amount of water obtainable from them. The method involves the injection into the well of steam in conjunction with hammering and thereby loosening particles of sand and gravel which are then removed by pumping.